Aquarium filter



FIG. 1 is a top plan view of an aquarium filter showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a right side elevational view thereof, the left side being a mirror image of the right side shown;

FIG. 4 is a rear elevational view thereof; and

FIG. 5 is a bottom view thereof. 

The ornamental design for an aquarium filter, substantially as shown and described. 